1. Field of the Invention
This invention relates to an adhesive composition for use in a flexible printed circuit board and a flexible printed circuit board by using the adhesive composition. More specifically, it relates to an adhesive composition which provides excellent adhesion strength between a base film and a copper foil and between a copper foil and a cover lay sheet and which can give a flexible printed circuit board having excellent soldering heat resistance; and to a flexible printed circuit board having the above excellent properties.
2. Prior Art
A flexible printed circuit board (sometimes abbreviated as FPC hereinafter) which is thin and lightweight and allows of three-dimensional packaging is becoming indispensable to electronic equipment which is increasingly becoming smaller in size, lighter in weight and higher in density in recent years.
Meanwhile, FPCs are also becoming higher in density and production of four to six layer FPCs is now possible. Because of this, FPC production process are becoming more and more complicated.
Both gloss surface and mat surface of a copper foil used in an FPC are generally subjected to a rustproof treatment. Since the copper foil comes in contact with various chemicals as the production process becomes more complicated, a rustproof coating film on the gloss surface of the copper foil, in particular, may be sometimes lost.
Or, there are cases where the rustproof coating film is removed so as to plate a through-hole or the like with copper. The copper surface from which the rustproof coating film has been removed is extremely chemically unstable and is easily corroded due to the presence of oxygen and carbon dioxide when it is exposed to humid air. When a copper circuit having corroded surfaces is covered with a cover lay sheet formed of a heat resistant film provided with an adhesive, reliabilities of adhesion strength and soldering heat resistance which are important properties for FPCs greatly decline. Therefore, when a copper circuit having no rustproof coating film is to be covered with a cover lay sheet, the surface of the copper circuit needs to be polished or softly etched to form a fresh copper surface.
Furihata, one of the co-inventors of the present invention, has already proposed adhesive compositions containing an epoxy resin, a carboxyl group-containing nitrile rubber and a curing agent (see U.S. Pat. No. 4,465,542, JP-A-60-79079 and JP-A-5-287255).
The adhesive compositions proposed by Furihata use an aromatic diamine, dicyandiamide, acid anhydride or imidazole derivative as the curing agent. However, it is not proposed that both an aromatic diamine and dicyandiamide are used in a specific ratio.
The adhesive compositions proposed by Furihata can bond a copper foil circuit having a rustproof coating film to a cover lay sheet with excellent adhesion strength and exhibit excellent soldering heat resistance after adhesion. However, it is desired that adhesive compositions can also bond a copper foil circuit without a rustproof coating film to a cover lay sheet while maintaining excellent adhesion strength and excellent soldering heat resistance, as described above, even if the copper surface is clean.